Drag conveyors of the general type disclosed herein have been known for a considerable period. Typically, such a conveyor comprises an elongated U-shaped trough having an inlet for material at one end and an outlet at the other, and sometimes, intermediate gatable outlets connected to various machines. The U-shaped bottom directs material toward the center, tending to make the unit self-cleaning. Such a capability is a requirement where product contamination is unacceptable upon changing from one product to another. After exiting from the outlet, the material is either processed or is directed by another conveyor to a further location. A link chain moves within the trough in a continuous, vertical plane and passes around a pair of sprockets on horizontal shafts, one of which sprockets is motor driven. Paddles which have their outer edges shaped to correspond to the bottom of the U-shaped trough are spaced equidistantly along the chain. The paddles are directed perpendicularly or radially outward of the chain so that in its lower run, the paddles drag the material along the trough from the inlet toward the outlet. The trough bottom thus frictionally supports the outer edges of the paddles and lower run of the chain against gravity, but support for the upper or return of the chain and its paddles has been another matter. The conveyor is typically between ten and two hundred fifty feet in length, making it necessary to provide support for the return run of chain to prevent sag and its undesirable consequences. Without chain or paddle support, the return paddles would interfere with product being fed in the opposite direction to the outlet. This required the provision of auxiliary structural elements for support of the chain at intervals along the return run. Chain sagging is unacceptable for a number of reasons. Chain tension is a system of this sort must be kept relatively high. The longer the conveyor and chain, the greater the chain's weight, thus requiring increased chain tension. Also, if the distance between the upper and lower chain runs is desired to be kept to a minimum to maintain a low conveyor profile, and if the return run support obstructs flow of material toward the outlet, contact of the material with the obstruction reduces the productive capacity of the conveyor. Such contact can cause the return paddles to move the uppermost portion of the material oppositely to the intended flow direction, and in some cases, even damage the material.
To prevent chain sag, the most common solution has been to provide idler sprockets for supporting the return run at spaced intervals between the upper and lower chain runs. Such idler sprockets require cross shafts for their support, and the shafts typically required either set collars or bearings in the side walls of the trough. The use of the idlers and their supporting elements is not only costly, but, because of the potentially abrasive nature of certain materials conveyed, they required frequent, time-consuming and costly maintenance, as well. Additionally, the cross shafts and idler sprockets of conventional drag conveyors presented the aforementioned obstruction to the flow of material, reducing the conveyor's productive capacity. Horizontal paddle-supporting rails have also been used in place of idler sprockets to support the paddles of the chain return run on some occasions, but they too are subject to initial cost and maintenance problems, not only of the rails and their supporting structure, but also from localized wear of the paddles themselves as the rails can tend to wear notches into the paddles.
A further problem with both of the two above-mentioned U-trough structures was the inability of the paddles under certain circumstances to remain in proper carrying contact with the bottom of the trough when performing dragging material toward the outlet. They had a tendency to lift tip off the bottom in instances where the product has a high moisture content, e.g., 25% or more. The paddles were also prone to twisting and skewing about the chain during abnormal side thrust, such as an unintended infeed of material from only one side of the inlet. To combat the lifting and twisting, some users, mainly those who had no concern about product contamination, have been known to bolt angle iron holddown rails to the insides of the carrying trough immediately above the inner edges of the paddles. However, the solution to the lifting or twisting problems created additional problems of somewhat impeding material flow (and attendant reduced capacity), undesirable retention of product standing on ledges created by the angle iron rails and degradation of and damage to certain products such as popcorn kernels and industrial particles intended to be kept intact at delivery. It is believed that this version was used primarily where self-cleaning at the end of a run was not essential or where product degradation was of little concern.
One other type of drag conveyor finding restricted use had a flat bottom and vertical side walls. The rectangular paddles improved capacity, but the design had other problems, such as lifting above the bottom, skewing and the inability to self-clean at the end of a run. Product often remained at the outside corners of the trough, essentially limiting the conveyor to use in situations where self-cleaning was not a requirement. The lifting problem was solvable by using rails, but that, brought on other problems noted above.